Method of mounting electronic components

ABSTRACT

An electronic component mounting method for mounting a electronic component on a board, in which an Au bump provided at an electronic component is joined to a joining terminal formed on a board by using solder made of Sn or solder containing Sn and the electronic component is adhered to the board by means of thermosetting resin thereby to mount the electronic component on the board. The applied thermosetting resin is flown toward the outside by the lower surface of the electronic component, then a part of the solder particles contained within the thermosetting resin are made in contact with the side surfaces of the Au bumps which are heated to the temperature higher than the melting point of the solder and also another part of the solder particles are molten in a state of being sandwiched between the Au bumps and the electrodes. Thus, the diffusion of Sn into the Au bumps from the outside is promoted and so the density of Sn within the Au bumps can be increased. Further, the diffusion of Sn into the Au bump from a solder joining portion can be suppressed and so the generation of Kirkendall voids can be suppressed.

TECHNICAL FIELD

The present invention relates to an electronic component mounting methodof mounting electronic components each provided with gold (Au) bumps ona board.

BACKGROUND ART

As a method of mounting electronic components on a board, one mode hasbeen employed widely in which bumps provided at the electronic componentare joined to joining terminals formed on the board, respectively. Au,which is excellent in conductivity and never subjected to the surfacedeterioration due to oxidization, is widely used as the material of thebumps. As a method of joining bumps made of Au to joining terminals,there has been known a method using anisotropic conductive materialwhich is formed by mixing conductive particles into thermosetting resin.In this method, the anisotropic conductive material is applied inadvance on the board so as to cover the joining terminals prior to themounting of the electronic components, and then the bumps are heatedwhile being pressed against the joining terminals at the time ofmounting the electronic components. Thus, the bumps and the joiningterminals are electrically made conductive therebetween via theconductive particles and further the main bodies of the electroniccomponents are joined with the board by the hardened anisotropicconductive material.

This method of using the anisotropic conductive material has anadvantage that both the electrical conductivity and the joining of themain bodies of the electronic components to the board can be performedsimultaneously. However, in order to secure good conductivity andrealize a low coupling resistance, this method is required to keep thecontacting state between the bump and the joining terminal uniform as toall of the bumps. To this end, the part mounting method using theanisotropic conductive material requires a mounting equipment with ahigh mounting accuracy. Thus, there has been required a method which canmount electronic components each provided with Au bumps easily and witha high quality.

Therefore, in order to satisfy such a requirement, various methods havebeen proposed in each of which the conductive particles contained in theanisotropic conductive material is replaced by solder particles (seepatent documents 1 to 4, for example). In each example of these patentdocuments, a sheet-shaped anisotropic conductive material formed bymixing the solder particles in resin is used, and at the time ofmounting electronic components, the solder resin disposed between bumpsand joining terminals are molten thereby to secure good conductivity.

-   [Patent Document 1] JP-A-8-186156-   [Patent Document 2] JP-A-10-112473-   [Patent Document 3] JP-A-11-4064-   [Patent Document 4] JP-A-11-176879

However, in the method shown in each of the aforesaid examples, when Susolder having been widely used in recent years as the solder materialconstituting the solder particles is employed, there arises a problemthat the intensity is degraded due to Kirkendall voids as explainedbelow. That is, in an area near the joining boundary where the bump madeof Au and the joining terminal are joined via the solder, the diffusionof Sn into Au proceeds with the lapse of time. In this case, in thesolder joining using the anisotropic conductive material, since anamount of the solder particles contained in the anisotropic conductivematerial is smaller as compared with that used in the usual solderjoining, the difference of density of Sn likely becomes large betweenthe solder joining portion and the bump. As a result, the diffusion ofSn from the solder joining portion into the bump is promoted.Accordingly, fine voids occurs at portions of the solder joining portionwhere Sn diffuses into Au, which causes the large degradation of thejoining intensity.

DISCLOSURE OF INVENTION

Accordingly, an object of the invention is to provide, in an electroniccomponent mounting method of joining bumps of Au by using solder, anelectronic component mounting method which can prevent the occurrence ofdegradation of intensity due to the Kirkendall voids.

An electronic component mounting method according to the invention isarranged in a manner that in the electronic component mounting method inwhich an Au bump provided at an electronic component is joined to ajoining terminal formed on a board by using solder made of Sn or soldercontaining Sn and the electronic component is adhered to the board bymeans of thermosetting resin thereby to mount, the electronic componenton the board, the method includes:

a resin application process of applying thermosetting resin, whichcontains solder particles that are formed by particulating the solder,on a region containing the joining terminal on the surface of the board;

a positioning process of performing positioning between the Au bump andthe joining terminal in a state that the electronic component is held bya holding head having a heating function;

a bump joining process of lowering the holding head while heating theelectronic component by the holding head thereby to join the Au bump tothe joining terminal; and

a resin hardening process of thermally curing the thermosetting resinthereby to adhere the electronic component on the board, wherein

in the bump joining process, the applied thermosetting resin is flowntoward the outside by the lower surface of the electronic component, thesolder particles contained in the thermosetting resin are made incontact by the holding head with the side surface of the Au bump heatedto a temperature higher than the melting point, of the solder, and apart of the solder particles are molten in a state of being sandwichedbetween the Au bump and the joining terminal.

According to the invention, the applied thermosetting resin is flowntoward the outside by the lower surface of the electronic component,then a part of the solder particles contained within the thermosettingresin are made in contact with the side surfaces of the Au bumps whichare heated to the temperature higher than the melting point of thesolder and also another part of the solder particles are molten in astate of being sandwiched between the Au bumps and the electrodes. Thus,the diffusion of Sn into the Au bumps from the outside is promoted andso the density of Sn within the Au bumps can be increased. Further, thediffusion of Sn into the Au bump from a solder joining portion can besuppressed and so the generation of Kirkendall voids can be suppressed.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1( a) to 1(c) are diagrams for explaining an electronic componentmounting method according to an embodiment of the invention.

FIGS. 2( a) and 2(b) are diagrams for explaining the electroniccomponent mounting method according to the embodiment of the invention.

FIGS. 3( a) and 3(b) are diagrams for explaining the electroniccomponent mounting method according to the embodiment of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the invention will be explained with reference to thedrawings. According to this electronic component mounting method, Aubumps provided at an electronic component are joined to joiningterminals formed on a board by using Sn solder or solder containing Sn,respectively, and the electronic component is adhered to the board bymean of thermosetting resin.

In FIG. 1( a), electrodes 2 each serving as a joining terminal areformed on the surface of the board 1. As shown in FIG. 1( b),thermosetting resin 3 containing solder particles (see solder particles4 shown in FIG. 3( a)) formed by particulating solder is applied on aregion containing the electrodes 2 at the surface of the board 1 (aresin application process). The solder used for the solder particles 4is Sn—Ag-basis solder containing Sn and Ag, Sn—Ag—Cu-basis soldercontaining Sn, Ag and Cu, or Sn—Bi-basis solder containing Sn and Bi.

Next, an electronic component 6 is mounted on the board 1 on which theresin has been applied. As shown in FIG. 1( c), each of electrodes 6 aformed on the lower surface of the electronic component 6 is providedwith an Au bump 5 having a convex portion 5 a at the lower portionthereof. The electronic component 6 is sucked and held by a holding head7 which has a heating function. The heating of the electronic component6 is started prior to the mounting on the board 1. The holding head 7thus holding the electronic component 6 moves above the board 1 whileheating the electronic component 6 and performs the positioning betweenthe Au bumps 5 and the electrodes 2, respectively (a positioningprocess).

Thereafter, the heating of the electronic component 6 is proceeded andwhen the temperature of the Au bumps 5 becomes higher than the meltingpoint of the solder particles 4 contained in the thermosetting resin 3,the placing operation of the electronic component 6 is started. That is,the holding head 7 is lowered thereby to join the Au bumps 5 to theelectrodes 2, respectively, while the holding head 7 heats theelectronic component 6 (a bump joining process). In this case, at first,as shown in FIG. 2( a), according to the lowering of the electroniccomponent 6, the convex portions 5 a of the Au bumps 5 are placed on theelectrodes 2, respectively, while extending the thermosetting resin 3applied on the surface of the board 1 toward the outside by force. Then,the holding head 7 continues to press and heat the electronic component6, whereby, as shown in FIG. 2( b), the Au bumps 5 are joined to theelectrodes 2 respectively in a state that the convex portions 5 a areslightly deformed. Simultaneously, the thermosetting resin 3 isthermally cured thereby to fix the electronic component 6 on the board 1(a resin hardening process).

FIGS. 3( a) and 3(b) show the behavior of the thermosetting resin 3 inthe aforesaid bump joining process. In the process of lowering theelectronic component 6 with respect to the board 1, firstly the convexportions 5 a of the Au bumps 5 are buried within the thermosetting resin3 and then the lower surface of the electronic component 6 pushes thethermosetting resin 3 down. Thus, the thermosetting resin 3 is extendedby force toward the outside from the inside, whereby the thermosettingresin 3 flows outside (a direction shown by an arrow a) together withthe solder particles 4 contained therein.

When the electronic component 6 is further lowered in this state, asshown in FIG. 2( b), a part of the solder particles 4 within thethermosetting resin 3 contact to the side surfaces of the Au bumps 5 andanother part of the solder particles are sandwiched between the lowersurfaces of the Au bumps 5 and the electrodes 2. These part and anotherpart of the solder particles 4 melt since they contact to the Au bumps 5having been heated to the temperature higher than the melting point ofthe solder particles 4 in advance. In this case, Sn constituting thesolder particles 4 diffuses into Au of the Au bumps 5 from the solderparticles 4 that contact to the side surfaces of the Au bumps 5 and arethus molten. The solder particles 4 molten between the Au bumps 5 andthe electrodes 2 are cooled and solidified later and so serve to jointhe Au bumps 5 to the electrodes 2, respectively.

In other words, according to the electronic component mounting method ofthe embodiment, the applied thermosetting resin 3 is flown outside bythe lower surface of the electronic component 6, then the part of thesolder particles 4 contained within the thermosetting resin 3 are madein contact with the side surfaces of the Au bumps 5 which are heated tothe temperature higher than the melting point of the solder by theholding head 7 and also the another part of the solder particles 4 aremolten in the state of being sandwiched between the Au bumps 5 and theelectrodes 2.

When the aforesaid electronic component mounting method is employed atthe time of joining the Au bumps 5 to the electrodes 2 by using thesolder containing Sn, the following excellent technical effects can beobtained. That is, in the case of using the solder containing Sn inorder to join the electronic components 6 to the electrodes 2, therearises a problem that the intensity is degraded due to the Kirkendallvoids. Concretely, in an area near the joining boundary where the bumpmade of Au and the electrode are joined via the solder, the diffusion ofSn from the solder joining portion into Au likely generates fine voids,which causes the degradation of the joining intensity.

In contrast, according to the electronic component mounting method ofthe embodiment, the solder particles 4 within the thermosetting resin 3are flown and made in contact with the side surfaces of the heated Aubumps 5 thereby to melt these solder particles 4 on the surface of theAu bumps 5. Thus, Sn within the solder particles 4 diffuses into the Aubump 5 from the outer portion toward the inner portion thereof and sothe density of Sn within the Au bumps 5 increases. This means that adifference between the density of Sn within the Au bump 5 and thedensity of Sn near the joining boundary between the Au bump 5 and theelectrode 2 reduces.

Thus, the diffusion of Sn into the Au bump 5 from the solder particles 4existing at the joining boundary between the Au bump 5 and the electrode2 can be suppressed, whereby the degradation of the joining intensitydue to the generation of the Kirkendall voids caused by the diffusion ofSn can be suppressed. In this case, since the Au bumps 5 are heated tothe temperature higher than the melting point of the solder particles 4before the Au bumps 5 are made in contact with the thermosetting resin3, the solder particles 4 made in contact with the Au bumps 5 arequickly molten and so the diffusion of Sn into the Au bumps 5 from theoutside thereof can be promoted.

This application is based upon and claims the benefit of priority ofJapanese Patent Application No. 2005-288107 filed on Sep. 30, 2005, thecontents of which are incorporated herein by reference in its entirety.

INDUSTRIAL APPLICABILITY

The electronic component mounting method according to the invention hasthe effect that the generation of the Kirkendall voids can be suppressedwhile also suppressing the diffusion of Sn into the Au bumps from thesolder joining portion and so can be applied to a usage in which Aubumps are joined to joining terminals formed on a board, respectively,by using solder made of Sn or solder containing Sn.

1. An electronic component mounting method for mounting an electroniccomponent on a board, in which an Au bump provided at an electroniccomponent is joined to a joining terminal formed on a board by usingsolder made of Sn or solder containing Sn and the electronic componentis adhered to the board by means of thermosetting resin thereby to mountthe electronic component on the board, comprising: a resin applicationprocess of applying thermosetting resin, which contains Sn-containingsolder particles that are formed by particulating the solder, on aregion containing the joining terminal on a surface of the board; apositioning process of performing positioning between the Au bump andthe joining terminal in a state that the electronic component is held bya holding head having a heating function; a bump joining process oflowering the holding head while heating the electronic component by theholding head thereby to join the Au bump to the joining terminal; and aresin hardening process of thermally curing the thermosetting resinthereby to adhere the electronic component on the board, wherein in thebump joining process, the applied thermosetting resin is pushed down bya lower surface of the electronic component to flow outside, the solderparticles contained in the thermosetting resin are made in contact bythe holding head with a side surface of the Au bump heated to atemperature higher than a melting point of the solder, and a part of thesolder particles are molten in a state of being sandwiched between theAu bump and the joining terminal.
 2. An electronic component mountingmethod according to claim 1, wherein the solder is Sn—Ag soldercontaining Sn and Ag, Sn—Ag—Cu solder containing Sn, Ag and Cu, or Sn—Bisolder containing Sn and Bi.
 3. An electronic component mounting methodaccording to claim 1, wherein the Au bump is heated to a temperaturehigher than the melting point of the solder before the Au bump is madein contact with the thermosetting resin.
 4. An electronic componentmounting method according to claim 2, wherein the Au bump is heated to atemperature higher than the melting point of the solder before the Aubump is made in contact with the thermosetting resin.